1. Field of the Invention
The present invention relates to apparatus and a method for generating nuclear magnetic resonance. More particularly, this invention pertains to improved optical pumping of a nuclear magnetic resonance cell.
2. Description of the Prior Art
The utilization of nuclear magnetic resonance (hereinafter referred to as "NMR") in a gyroscope is disclosed in U.S. Pat. No. 4,157,495 which issued June 5, 1979 and which is assigned to the same assignee as the present invention.
The gyroscope disclosed therein operates on the principle of sensing inertial angular rotation rate or angular displacement about a sensitive axis as a shift in the Larmor precession frequency or phase, respectively, of one or more isotopes that possess nuclear magnetic moments.
The gyroscope is composed of an angular rotation sensor and associated electronics. The principal elements of the sensor are a light source, an NMR cell, a photodetector, a set of magnetic shields and a set of magnetic field coils. The principal elements of the electronics are signal processing circuits for extracting the Larmor precession frequency and phase information as well as circuits for generating and controlling various magnetic fields, both steady and varying sinusoidally with time, that are necessary for the proper operation of the device.
The NMR cell is mounted within a set of magnetic shields in order to attenuate external magnetic fields to acceptable low levels. Magnetic field coils are used to apply uniform magnetic fields to the NMR cell. Both a steady field and an a.c. carrier field are applied along the sensitive axis of the device and an a.c. feedback field is applied along one of the transverse axes. The d.c. magnetic fields along both transverse axes are controlled to be substantially zero. The NMR cell contains a single alkali metal vapor, such as rubidium, together with two isotopes of one or more noble gases, such as krypton or xenon. One or more buffer gases such as helium or nitrogen may also be contained in the cell.
As disclosed in the patent, the NMR cell is illuminated by a beam of circularly polarized light that originates from a rubidium lamp and thereafter passes through the cell at an angle with respect to the steady magnetic field. Absorption of some of this light causes the atomic magnetic moments of the rubidium atoms to be partly aligned in the direction of the steady magnetic field. This alignment is partly transferred to the nuclear magnetic moments of the noble gases, and these moments are caused to precess about the direction of the steady magnetic field, which, in turn, creates magnetic fields that rotate at the respective Larmor precession frequencies of the two noble gases. These rotating fields modulate the precessional motions of the rubidium magnetic moments, which in turn produce corresponding modulations of the transmitted light, thereby making it possible to detect the Larmor precession frequencies of the two noble gases optically.
The modulations of the light intensity are converted into electrical signals by a photodetector, and these signals are then electronically demodulated and filtered to provide signals at the Larmor precession frequencies of the two noble gases. The difference between the two precession frequencies is used to control the steady magnetic field so that it is substantially constant. One of the noble gas precession frequencies is subtracted from a precision reference frequency and the resulting difference frequency serves as a measure of the angular rotation rate of the gyroscope. The magnitude of an individual nuclear magnetic moment is extremely small, the natural equilibrium condition being one in which a nearly random orientation of moments exists in an ensemble of atoms. Techniques are employed to orient a significant fraction of such magnetic moments in a single direction so that a macroscopic magnetic moment, and consequently a measurable signal, will be produced.
The aligned magnetic moments of the single alkali metal system and of both noble gas systems of atoms are subject to relaxation mechanisms tending to cause their alignment to decay with time toward a random equilibrium orientation. Each system of moments is characterized by an individual relaxation time constant that depends upon the kinds and quantities of all constituents and upon the total environment within the cell. The steady state fractional alignment of each system of moments is a function of both the optical pumping rate and the relaxation time for the system.
The r.f.-powered alkali metal vapor lamp utilized for optical pumping and magnetometric detection in an NMR gyroscope of the type described above, when properly tuned, produces an output frequency spectrum that closely approximates the absorption spectrum of the rubidium vapor within the NMR cell. Such matching of spectra is inherent in the use of the same alkali metal vapor as both an emission medium and as an absorption medium.
While the r.f.-powered alkali metal vapor lamp has provided acceptable optical pumping of the NMR cell, this type of light source is characterized by a relatively large size, weight and power consumption. Such characteristics have, in fact, rendered the NMR gyroscope unsuitable for some applications and have rendered the device only marginally effective in others.
One optical source that possesses favorable operational characteristics vis a vis the vapor lamp in the areas of size, weight and power consumption is the laser diode. In addition to the relative savings in space and power requirements effected by such device, the use of laser light introduces the advantage of frequency tunability and high beam collimation into the optical pumping process.
A disadvantage associated with the use of laser, as opposed to vapor lamp, light results from its inherently narrow bandwidth. While a highly advantageous property for many applications, the narrow bandwidth of laser emissions limits the amount of NMR signal that can be generated by failing to utilize a significant portion of the absorption spectrum of the rubidium (or other alkali metal) vapor within the NMR cell.